1. Field of the Invention
The present invention relates to a method of and an apparatus for processing an image and, more particularly, to an image processing method and an apparatus for the same which execute effective tone correction while a still image represented by a video signal is recorded in an image recording medium to provide a visible image.
2. Description of the Prior Art
In the imaging art, there has been proposed a still image recording apparatus which receives a video signal read out of a video floppy disk, a video tape and other video signal recording media so as to reproduce a visible image in an image recording medium such as a printing paper. In this type of apparatus, a red (R), a green (G) and a blue (B) color signal, for example, which are produced from input video signals are sequentially fed to a high luminance black-white cathode ray tube (CRT) which is exclusively adapted for recording. A lens and a color separating filter for separating three colors are positioned in front of the screen of the recording CRT, so that an image appearing on the screen may be focused onto a color printing paper. While a cyan (C), a magenta (M) and a yellow (Y) coloring matter in the printing paper are stimulated to color to provide a color image, a prerequisite is that the color signals R, G and B derived from the individual input video signals and the signals applied to the recording CRT for stimulating the C, M and Y coloring matters be inversely related to each other with respect to the level. For example, when the levels of the color signals R, G and B are relatively high representing a light portion of a picture, the coloring degree of the C, M and Y coloring matters has to be suppressed to render the lightness and, therefore, the outputs associated with the respective color signals R, G and B have to be lowered in level.
Another prerequisite with the above-described type of apparatus is that the difference between the density of each color of an image as represented by input video signals and those of a actual image be compensated for, the difference being ascribable to the lighting, the kind of a camera used, and other photographing conditions. For example, it sometimes occurs that incoming color signals R, G and B are different in maximum level from each other, despite that an actual image includes a white portion where the colors R, G and B are equal in intensity and maximum in level. In such a condition, should the input color signals R, G and B be fed out without any compensation, a portion which should be rendered, for example, in white would be recorded in a recording medium in a color which is close to any of the three colors, due to the difference between the levels of the color signals R, G and B. To compensate for the different levels of input color signals R, G and B, tone correction is applied to the signals R, G and B by using look-up tables each of which contains output level data associated with a respective one of the signals R, G and B. The previously stated conversion of signal levels is effected by such tone correction.
The data in the look-up tables are such that when an image includes a white portion, a highlight and a shadow point representative of a maximum and a minimum level, respectively, are determined for each of the input color signals R, G and B, and the output levels of the individual color signals R, G and B corresponding to the highlight and shadow points are caused to agree with each other. This allows the color signals R, G and B representative of the white portion to be delivered with equal intensity, whereby the white portion is reproduced in white.
The highlight and shadow points may be determined by preparing a cumulation histogram which shows a frequency distribution of video signals of respective pixels of an input video signal which constitute one frame of image. Then, a point where the cumulation histogram shows, for example, 99% is determined to be the highlight point. Specifically, an input video signal of a particular level at which the proportion of those pixel signals which are lower than that level to all of the pixel signals is 99% is determined to be the highlight point.
In a tone correcting method known in the art, a cumulation histogram is produced by using a luminance signal Y of input video signals, then a highlight and a shadow point are set up on the basis of that cumulation histogram, and then look-up tables for the tone correction of respective color signals R, G and B are prepared by applying those highlight and shadow points to all of the three color signals R, G and B. This kind of tone correcting method is usable under a so-called low correction condition. This method, however, is incapable of compensating for the difference in tone between input video signals and an actual image as previously mentioned, because it does not give consideration to the difference between the levels of the color signals R, G and B of the input video signals.
Another approach for tone correction, which is applicable to so-called high correction, consists in preparing one cumulation histogram for each of the color signals R, G and B of input video signals, then determining a highlight and a shadow point based on each of such histograms, and then producing one look-up table for each of the color signals R, G and B. A problem with this approach is as follows. Assuming that the color signal R, for example, includes a portion of significantly high level representative of a light image portion, the highlight point of the signal R is selected to be higher than those of the other color signals G and B resulting that the output corresponding to the signal R becomes weak to render cyan (C), which is complementary to red (R), dark in a reproduced picture and, thereby, disturbs the color balance. Specifically, while no problem arises so long as an image represented by input video signals includes a perfect white portion, the above-stated undesirable phenomenon occurs when it does not include a perfect white portion while, at the same time, any of the input signals is representative of a pure color of high chroma and high level; the image subjected to tone correction which uses look-up tables would appear disturbed in color balance. That is, the image would undergo overcorrection to be thereby reproduced with different tones from those of the actual image.
It follows from the above that adequate tone correction is unattainable unless the low correction, high correction and a certain degree of correction between the high and low correction are effected alternatively depending on the image which is represented by input video signals. Such adjustment of correction level has customarily relied on operator's eyesight. For example, while watching an image represented by tone-corrected video signals on a monitor, the operator selects high correction when decided that the image includes a perfect white portion and selects low correction when decided that the image does not include a white portion and includes a portion of a color, other than white, which is high in chroma and light. However, although the operator watching the monitor may decide that the image includes a white portion, it is difficult for the operator to determine whether or not that white portion is highlight white, i.e., perfect white represented by the highest levels of the color signals R, G and B. It is also difficult for the operator to see if the image includes a portion of a color, other than white, which has high chroma. This kind of adjustment, therefore, not only consumes a disproportionate period of time but also needs skilled labor.